The present invention relates to the focusing of laser light emitted by laser instruments and, in particular, to the use of a weak focusing lens to improve the focus and alignment of a plane of laser light reflected from a cone in a laser instrument.
In many laser instruments a collimating lens or nearly collimating lens is used with a small source of laser light, such as a laser diode, to produce a beam of laser light which is either substantially collimated or focused to a distant focal point in the far field. In addition to producing beams of laser light, laser instruments also produce planes of laser light. In one such laser instrument 1, shown in part in FIG. 1A, a suspended, reflective cone 2 is used to reflect an axially oriented beam 3 of laser light 4 to produce a plane 5 of laser light. Where the reflective cone 2 is centered in the beam 3, the reflected laser light 4 emanates radially as a plane 5 in all directions with substantial uniformity. In many such laser instruments 1 the incident beam 3, and thus the reflected plane 5 of laser light 4, have a distant focal point.
Where reflective cones are used to produce planes of laser light, the angle at which laser light rays strike the cone is critical to the production of a level plane of light. Even slight changes in the angle at which the laser light rays strike the reflective cone will cause an umbrella-like shift in the plane of laser light produced, causing the plane to have a slightly conical shape, as also shown in FIG. 1A. Thus, it has been found that proper beam focus is needed to produce a level plane of laser light.
Laser diodes are typically used as a small laser light source 6 in such laser instruments 1. Ideally, the laser diode is at the design back focus distance of a collimating lens or nearly collimating lens 7, so that the intended beam of collimated or slightly focused laser light will be produced. However, where the separation between the laser diode and lens 7 varies from the design back focus distance, the angle at which laser light rays strike the reflective cone varies, and a large change in the planarity of the reflected light can result. It has been found that a change of 1 arc second, i.e. 1/3600 of 1 degree, in the angle of the incoming laser light rays is sufficient to produce the umbrella-like shift to an objectionable degree in the plane of laser light shown in FIG. 1A.
Several sources of such variation exist, but have been overcome by adherence to tight manufacturing tolerances. For example, during assembly of such laser instruments, mounting errors can cause the distance between the laser diode and collimating lens to vary. As well, each laser diode lases at a spot which cannot be exactly predetermined in all three dimensions. This variation from laser diode to laser diode effects the ability to mount the laser diode at the design back focus distance of the collimating lens. In addition, the curvature of the collimating lens surfaces can vary within a tolerance from lens to lens, causing the actual back focus distance to vary from the design back focus distance. To control the effect of these sources of variation, in the assembly of laser instruments, the distance from the laser source to the collimating lens has been tightly controlled to tolerances up to .+-.0.0003 inches.
As well, as there is variation from laser diode to laser diode in the spot at which the diodes lase, there is a limit to how closely the laser diode can be located on the focal axis 8 of the collimating lens or nearly collimating lens 7. As shown in FIG. 1B, where like numerals represent like elements, axial misalignment of the laser diode can also cause a tilting of the plane of laser light produced.
Because a very slight change in the back focus distance can have a critical effect on the focus of the beam and planarity of the plane produced, even after careful assembly, laser instruments using reflective cones require further testing and adjustment. The position of either the collimating lens or laser diode, and the separation therebetween is adjusted, as needed, to produce a plane of laser light having the desired orientation and planarity.
After installation in laser instruments, laser diodes occasionally burn out during use and need to be replaced. Since there is no adjustment of the collimating lens after initial installation and there is variation from laser diode to laser diode in the spot at which the diodes lase, there is a limit to how accurately the new laser diode can be matched to the design back focus distance of the collimating lens. Due to the sensitivity of reflected planes of laser light to the beam focus, planarity of the laser light plane produced by the repaired laser instrument can be effected, as shown in FIG. 1A. As well, there remains a limit to how closely the new laser diode can be located on the focal axis, and axial misalignment of the laser diode can, again, cause a tilting of the plane of laser light produced by the repaired laser instrument, as shown in FIG. 1B. Thus, when laser diodes are replaced, it becomes likely that the new beam produced in the laser instrument will be slightly out of focus and slightly off axis, causing a variation in both the planarity and the angular orientation of the plane of laser light produced.
Despite the necessity for careful manufacturing techniques, laser diodes have been successfully used as laser sources in a variety of laser instrument applications. However, the need exists for greater ease and flexibility in controlling the planes of light produced from laser diodes in laser instruments, and for replacement of laser diodes in a manner which will compensate for their inherent variability.